Methods for forming nanotube fabric layers with increased density

ABSTRACT

Methods for passivating a nanotube fabric layer within a nanotube switching device to prevent or otherwise limit the encroachment of an adjacent material layer are disclosed. In some embodiments, a sacrificial material is implanted within a porous nanotube fabric layer to fill in the voids within the porous nanotube fabric layer while one or more other material layers are applied adjacent to the nanotube fabric layer. Once the other material layers are in place, the sacrificial material is removed. In other embodiments, a non-sacrificial filler material (selected and deposited in such a way as to not impair the switching function of the nanotube fabric layer) is used to form a barrier layer within a nanotube fabric layer. In other embodiments, individual nanotube elements are combined with and nanoscopic particles to limit the porosity of a nanotube fabric layer.

This application is a continuation of U.S. patent Ser. No. 15/061,255filed Mar. 4, 2016 and entitled “Methods for Passivating a CarbonicLayer,” which claims benefit and is a continuation of U.S. patentapplication Ser. No. 14/535,675 filed Nov. 7, 2014 and entitled “Methodsfor Passivating a Carbonic Layer,” which claims benefit and is acontinuation of U.S. patent application Ser. No. 14/020,095 filed Sep.6, 2013 and entitled “Methods for Passivating a Carbonic Layer,” whichclaims benefit of and is a divisional patent application of U.S. patentapplication Ser. No. 12/910,714 filed Oct. 22, 2010 and entitled“Methods for Passivating a Carbonic Nanolayer,” which claims the benefitof U.S. Provisional Patent Application No. 61/254,588 filed Oct. 23,2009 and entitled “Methods for Passivating a Nanotube Fabric Layer byControlling the Density of the Nanotube Fabric Layer,” the entirecontents of each of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to nanotube based switching elements, andmore particularly to methods of forming a passivated layer within thenanotube fabric layer of said switching elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following U.S. patents, which areassigned to the assignee of the present application, and are herebyincorporated by reference in their entirety:

Methods of Nanotube Films and Articles (U.S. Pat. No. 6,835,591), filedApr. 23, 2002;

Methods of Using Pre-Formed Nanotubes to Make Carbon Nanotube Films,Layers, Fabrics, Ribbons, Elements, and Articles (U.S. Pat. No.7,335,395), filed Jan. 13, 2003;

Spin-Coatable Liquid for Formation of High Purity Nanotube Films (U.S.Pat. No. 7,375,369), filed Jun. 3, 2004.

This application is related to the following patent applications, whichare assigned to the assignee of the application, and are herebyincorporated by reference in their entirety:

Methods of Making Carbon Nanotube Films, Layers, Fabrics, Ribbons,Elements, and Articles (U.S. patent application Ser. No. 10/341,005),filed Jan. 13, 2003;

High Purity Nanotube Fabrics and Films (U.S. patent application Ser. No.10/860,332), filed Jun. 3, 2004;

Two terminal Nanotube Devices and Systems and Methods of Making Same(U.S. patent application Ser. No. 11/280,786), filed Nov. 15, 2005;

Nanotube Articles with Adjustable Electrical Conductivity and Methods ofMaking the Same (U.S. patent application Ser. No. 11/398,126), filedApr. 5, 2006;

Nonvolatile Nanotube Diodes and Nonvolatile Nanotube Blocks and SystemsUsing Same and Methods of Making Same (U.S. patent application Ser. No.11/835,651), filed Aug. 8, 2007;

Nonvolatile Resistive Memories Having Scalable Two terminal NanotubeSwitches (U.S. patent application Ser. No. 11/835,612), filed Aug. 8,2007;

Nonvolatile Nanotube Diodes and Nonvolatile Nanotube Blocks and SystemsUsing Same and Methods of Making Same (U.S. patent application Ser. No.11/835,856), filed Aug. 8, 2008;

Memory Elements and Cross Point Switches and Arrays of Same UsingNonvolatile Nanotube Blocks (U.S. patent application Ser. No.12/511,779), filed Jul. 29, 2009;

Nonvolatile Nanotube Diodes and Nonvolatile Nanotube Blocks and SystemsUsing Same and Methods of Making Same (U.S. patent application Ser. No.12/273,807), filed Nov. 19, 2008;

Improved Switching Materials Comprising Mixed Nanoscopic Particles andCarbon Nanotubes and Methods of Making and Using Same (U.S. patentapplication Ser. No. 12/274,033), filed Nov. 19, 2008.

BACKGROUND OF THE INVENTION

Any discussion of the related art throughout this specification shouldin no way be considered as an admission that such art is widely known orforms part of the common general knowledge in the field.

Nanotube based switching devices offer a plurality of uses withincommercial electronics. Such nanotube based switching elements can beused as nonvolatile memory devices, combined to form logic devices, andused to form analog circuit elements such as, but not limited to,nanotube based field effect transistors and programmable power supplies.In particular, two terminal nanotube based switching devices arebecoming increasingly desirable within electronic systems—such as, butnot limited to, memory arrays, microprocessors, and FPGAs—and arrays ofsuch devices are continually increasing in complexity and density,creating a need for smaller and smaller individual devices.

SUMMARY OF THE DISCLOSURE

The current invention relates to the passivation of nanotube fabriclayers within two terminal nanotube switching devices.

In particular, the present disclosure provides a nanotube switchingdevice. This nanotube switching device comprises a first conductiveelement, a second conductive element, and a nanotube fabric layer whichincludes a plurality of individual nanotube elements. The nanotubefabric layer includes a first side and a second side, wherein the firstside of the nanotube fabric layer is electrical coupled to the firstconductive element and the second side of said nanotube fabric layer iselectrically coupled to the second conductive element. The density ofthe nanotube fabric layer is selected such as to limit the encroachmentof at least one of the first conductive element and the secondconductive element into the nanotube fabric layer.

The present disclosure also provides another nanotube switching device.This nanotube switching device comprises a first conductive element, asecond conductive element, a first nanotube fabric layer (which includesa first plurality of individual nanotube elements), and a secondnanotube fabric layer (which includes a second plurality of individualnanotube elements). The first nanotube fabric layer includes a firstside a second side, and the second nanotube layer also includes a firstside and a second side. Within this nanotube switching device, the firstside of the first nanotube fabric layer is electrically coupled to thefirst side of said second nanotube fabric layer; the second side of thefirst nanotube fabric layer is electrically coupled to the firstconductive element; and the second side of the second nanotube fabriclayer is electrically coupled to the second conductive element. Further,the density of the second nanotube fabric layer is selected as to limitthe encroachment of the second conductive element into the secondnanotube fabric layer and the density of the first nanotube fabric layeris selected as to optimize the switching operation of the first nanotubefabric layer.

The present disclosure also provides a method for forming a nanotubeswitching device. This method comprises first forming a nanotube fabriclayer over a first material layer, the nanotube fabric layer comprisinga plurality of individual nanotube elements. The method furthercomprises flowing a sacrificial material over the nanotube fabric layersuch that the sacrificial material penetrates the nanotube fabric layerand forms a barrier layer within the nanotube fabric layer. The methodfurther comprises depositing a second material layer adjacent to thenanotube fabric layer. The method further comprises volatizing andremoving at least a portion of the barrier layer.

The present disclosure also provides another method for forming ananotube switching device. This method comprises forming a nanotubefabric layer over a first material layer, the nanotube fabric layercomprising a plurality of individual nanotube elements. The methodfurther comprises flowing a filler material over the nanotube fabriclayer such that the filler material penetrates the nanotube fabric layerand forms a barrier layer within the nanotube fabric layer. The methodfurther comprises depositing a second material layer adjacent to thenanotube fabric layer.

The present disclosure also provides another method for forming ananotube switching device. This method comprises combining a firstvolume of individual nanotube elements and a second volume of nanoscopicparticles in a liquid medium to form an application solution. The methodfurther comprises depositing the application solution over a firstmaterial layer as to form a composite switching layer, this compositelayer comprising a mixture of individual nanotube elements andnanoscopic particles. The method further comprises depositing a secondmaterial layer adjacent to the composite switching layer.

The present disclosure also provides another method for forming ananotube switching device. This method comprises combining a firstvolume of individual nanotube elements and a second volume of nanoscopicparticles in a liquid medium to form an application solution. The methodfurther comprises depositing the application solution over a firstmaterial layer as to form a composite switching layer, this compositelayer comprising a mixture of individual nanotube elements andnanoscopic particles. The method further comprises flowing a fillermaterial over the composite switching layer such that the fillermaterial penetrates the composite switching layer and forms a barrierlayer within the composite layer. The method further comprisesdepositing a second material layer adjacent to said composite switchinglayer.

Accordingly it is the object of the present disclosure to provide aplurality of methods for passivating a nanotube fabric layer.

It is also an object of the present disclosure that these passivationmethods do not impede or otherwise adversely affect the switchingcharacteristics of a nanotube fabric layer in which they are employed.

It is further an object of the present disclosure that these passivationmethods be compatible within the fabrication of a two terminal nanotubeswitching device as taught by Bertin in Ser. No. 11/280,786.

It is also an object of the present disclosure that these passivationmethods allow for the fabrication of two terminal nanotube switchingdevices with nanotube fabric layers of thicknesses on the order of 20nm.

Other features and advantages of the present disclosure will becomeapparent from the following description of the disclosure which isprovided below in relation to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration depicting a first two terminal nanotubeswitching device comprising a nanotube fabric layer of thickness T₁;

FIG. 2 is an illustration depicting a second two terminal nanotubeswitching device comprising a nanotube fabric layer of thickness T₂;

FIG. 3 is an illustration depicting a two terminal nanotube switchingdevice comprising a low density nanotube fabric layer with a relativelyhigh porosity;

FIG. 4 is an illustration depicting a two terminal nanotube switchingdevice wherein the porosity of nanotube fabric layer has been selectedby limiting the length of the individual nanotube elements comprisingthe nanotube fabric layer;

FIG. 5 is an illustration depicting a two terminal nanotube switchingdevice wherein the porosity of nanotube fabric layer has been selectedthrough the use of a specific application method;

FIG. 6 is an illustration depicting a two terminal nanotube switchingdevice comprising multiple nanotube fabric layers;

FIGS. 7A-7J are a series of process diagrams which illustrate a methodof passivating a nanotube fabric layer through the use of a sacrificialfiller material;

FIGS. 8A-8K are a series of process diagrams which illustrate a methodof passivating a nanotube fabric layer through the use of a sacrificialfiller material wherein the sacrificial material is volatized andremoved through a non-hermetic material layer;

FIGS. 9A-9M are a series of process diagrams which illustrate a methodof passivating a nanotube fabric layer through the use of a sacrificialfiller material wherein the sacrificial material is volatized during theformation of the individual two terminal nanotube switching elementswithin an array;

FIGS. 10A-10D are a series of process diagrams which illustrate a methodof passivating a nanotube fabric layer through the use of anon-sacrificial filler material;

FIGS. 11A-11F are a series of process diagrams which illustrate a methodof passivating a nanotube fabric layer through the use of a porousdielectric material.

FIGS. 12A-12F are a series of process diagrams which illustrate a methodof passivating a nanotube fabric layer through the use of a roomtemperature chemical vapor deposition (RTCVD) process;

FIGS. 13A-13E are a series of process diagrams which illustrate a methodof passivating a nanotube fabric layer through the use of anon-directional sputter deposition process;

FIG. 14 is a process diagram illustrating a the formation of apassivated two terminal nanotube switching device which includes ananotube fabric layer which comprises both individual nanotube elementsand nanoscopic particles;

FIGS. 15A-15F is a process diagram illustrating a method of furtherpassivating a nanotube fabric layer comprised of individual nanotubeelements and nanoscopic particles through the use of a room temperaturechemical vapor deposition (RTCVD) process;

FIGS. 16A-16E is a process diagram illustrating a method of furtherpassivating a nanotube fabric layer comprising individual nanotubeelements and nanoscopic particles through the use of a non-directionalsputter deposition process.

DETAILED DESCRIPTION

U.S. patent application Ser. No. 11/280,786 to Bertin et. al,incorporated herein by reference, teaches the fabrication of such twoterminal nanotube switching devices. As taught by Bertin, a two terminalswitching device includes a first and second conductive terminals and ananotube article. The nanotube article overlaps a portion of each of thefirst and second conductive terminals. In at least some embodimentsdescribed by Bertin, the nanotube article is a nanotube fabric layerdisposed over the first conductive terminal. In such embodiments thesecond conductive terminal is disposed over the nanotube fabric layer,forming a three layer device with the nanotube fabric layersubstantially between the first and second conductive elements.

Bertin further describes methods for adjusting the resistivity of thenanotube fabric layer between a plurality of nonvolatile resistivestates. In at least one embodiment, electrical stimuli is applied to atleast one of the first and second conductive elements such as to pass anelectric current through said nanotube fabric layer. By carefullycontrolling these electrical stimuli within a certain set ofpredetermined parameters (as described by Bertin in Ser. No. 11/280,786)the resistivity of the nanotube fabric layer can be repeatedly switchedbetween a relatively high resistive state and relatively low resistivestate. In certain embodiments, these high and low resistive states canbe used to store a digital bit of data (that is, a logic “1” or a logic“0”).

FIG. 1 illustrates a first two terminal nanotube switching device 100.In certain embodiments, a nanotube fabric layer 130 (of thickness T₁) isdeposited over a first conductive element 110 in a first operation. In asecond operation, a second conductive element 120 is deposited over thenanotube fabric layer 130. As depicted in FIG. 1, as the secondconductive element 120 is applied, the second conductive element 120 mayseep partially into the porous nanotube fabric layer 130. Typically suchpenetration is not problematic or detrimental to the operation of twoterminal nanotube switching device 100 as the thickness (T₁) of thenanotube fabric layer 130 is significantly larger than the depth of thepenetration. That is, while second conductive element 120 seepspartially into porous nanotube fabric layer 130, the nanotube fabriclayer 130 is thick enough to prevent an electrical short between thefirst conductive element 110 and the second conductive element 120.

FIG. 2 illustrates a second two terminal nanotube switching device. Twoterminal nanotube switching device 200 (as depicted in FIG. 2) isintended to illustrate a switching device fabricated on a significantlysmaller scale as compared with the two terminal nanotube switchingdevice 100 depicted in FIG. 1. Nanotube fabric layer 230 (of thicknessT₂) is deposited over a first conductive element 210 in a firstoperation. In a second operation, a second conductive element 220 isdeposited over nanotube fabric layer 230. As with the two terminalnanotube switching device 100 depicted in FIG. 1, the second conductiveelement 220 may seep partially into the porous nanotube fabric layer230. However, in certain embodiments, as nanotube fabric layer 230 isrelatively thin, second conductive element 220 can substantiallypenetrates all the way through porous nanotube fabric layer 230,effectively forming an electrical short circuit between first conductiveelement 210 and second conductive element 220.

In one non-limiting example, T₁ (the thickness of the nanotube fabriclayer 130) is on the order of 100 nm, while T₂ (the thickness of thenanotube fabric layer 230) is on the order of 20 nm. In anothernon-limiting example, T₂ would be on the order of 5 nm. It should benoted that while these exemplary thickness values are intended toillustrate a specific fabrication issue as two terminal nanotubeswitching devices are realized on increasingly smaller scales, thisfabrication issue is not limited to these dimensions. Further, aplurality of factors may contribute to the depth of which a conductiveelement will penetrate into a nanotube fabric layer over which it isdeposited. Such factors include, but are not limited to, the density ofthe nanotube fabric layer, the method used to deposit the overlyingconductive layer, and the material used to form the overlying conductivelayer.

As described in the preceding discussion, FIG. 2 illustrates a potentiallimitation as the physical dimensions of two terminal nanotube switchesare reduced. Nanotube fabrics can be porous and can be susceptible topenetration by a material deposited over them. Further, the performanceof nanotube fabric layers within such switching devices can be degradedby certain materials, gases, or contaminants. For example, oxygen andwater can react with CNTs at high temperatures. Carbon nanotubes canalso be functionalized with other contaminating materials—such as, butnot limited to, fluorine and chlorine—which may also deteriorate theoperational parameters of a nanotube fabric layer by altering theelectrical properties of the individual carbon nanotubes elements withinthe fabric and carbon nanotube. Furthermore, without wishing to be boundby theory, within a two terminal nanotube switching device, thenanometer-level motion of the individual nanotube elements may desirablyremain unhindered by the surrounding device structure.

As such, certain embodiments of the present invention provides a methodof two terminal nanotube switch fabrication which provides a physicalbarrier (a passivation layer) adjacent to or within the nanotube fabriclayer such that adjacent material layers as well as other contaminantsare prevented from penetrating the nanotube fabric layer. Certainembodiments of the present invention further advantageously provide thismethod without interfering with or otherwise adversely affecting theswitching operation of the nanotube fabric layer.

The present disclosure involves the passivation of nanotube fabriclayers within the fabrication of two terminal nanotube switchingdevices. As will be shown in the following discussion of the presentdisclosure, nanotube fabric layers can be passivated—that is formed orprepared in such a way as to prevent or otherwise limit the encroachmentof an adjacent material layer—in a plurality of ways.

In some aspects of the present disclosure, a nanotube fabric layer canbe passivated by increasing the density of the nanotube fabric layer byadjusting one or more characteristics of the individual nanotubeelements contained in the nanotube fabric layer (e.g., length,orientation, etc.). This increased density limits the porosity of thenanotube fabric layer, thereby limiting the depth to which an adjacentmaterial layer can penetrate.

In some aspects of the present invention, a sacrificial material isimplanted within a porous nanotube fabric layer during the fabricationprocess. This sacrificial material is used to fill in the voids and gapswithin the porous nanotube fabric layer while one or more other materiallayers are applied adjacent to the nanotube fabric layer. Once the othermaterial layers are in place, the sacrificial material is removed,allowing the nanotube fabric layer to function.

In other aspects of the present disclosure, a non-sacrificial fillermaterial is used to passivate a nanotube fabric layer. Within theseaspects of the present disclosure, a filler material is selected anddeposited within a nanotube fabric layer in such a way as to notadversely affect the switching function of the nanotube fabric layer. Inthis way, a barrier layer is formed within the porous nanotube fabriclayer which prevents an adjacent material layer from fully penetratingthrough the nanotube fabric layer.

In other aspects of the present disclosure, a nanotube fabric layer isformed comprising a first plurality of individual nanotube elements anda second plurality of nanoscopic particles. The second plurality ofnanoscopic particles serves to limit the porosity of the nanotube fabriclayer, thereby limiting the encroachment of adjacent material layersinto the nanotube fabric layer. The nanoscopic particles are selectedand combined with the individual nanotube elements in such a way as tonot adversely affect the switching operation of the nanotube fabriclayer.

Each of these aspects will be described in the following sections inaccordance with the accompanying figures.

Within the methods of the present disclosure, nanotube fabric layers canbe formed over substrate elements. The methods include, but are notlimited to, spin coating (wherein a solution of nanotubes is depositedon a substrate which is then spun to evenly distribute said solutionacross the surface of said substrate), spray coating (wherein aplurality of nanotube are suspended within an aerosol solution which isthen disbursed over a substrate), and in situ growth of nanotube fabric(wherein a thin catalyst layer is first deposited over a substrate andthen used to form nanotubes). (See, e.g., U.S. Pat. No. 7,335,395 toWard et al., which is incorporated herein by reference in its entirety.)U.S. Pat. No. 7,375,369 to Sen et al., and U.S. Patent Publication No.2006/0204427, both of which are incorporated herein by reference in itsentirety, teach nanotube solutions which is well suited for forming ananotube fabric layer over a substrate element.

It should also be noted that while the figures within the presentdisclosure (as well as the accompanying description of those figures)depict substantially vertically oriented two terminal nanotube switchingdevices—that is, a nanotube fabric layer positioned between a firstconductive element below and a second conductive element above—themethods of the present disclosure are not limited in this regard.Indeed, as is described in the incorporated references (most notablyU.S. patent application Ser. No. 11/835,651 to Bertin et. al.,incorporated herein by reference in its entirety) two terminal nanotubeswitching devices can be formed within a plurality of orientations,including, but not limited to, vertical, horizontal, two dimensional(wherein a nanotube fabric layer is in contact with two or moreelectrode elements substantially in the same plane), and disposed overone or more flexible electrode elements. It will be clear to thoseskilled in the art that the methods of the present disclosure (asdescribed below with respect to substantially vertical two terminalswitching devices for the sake of clarity) are applicable to twoterminal nanotube switching devices constructed in any of theseorientations.

It should also be noted that while the figures within the presentdisclosure (as well as the accompanying description of those figures)describe the passivation methods of the present disclosure within thescope of the fabrication of two terminal nanotube switching devices, themethods of the present disclosure are not limited in this regard.Indeed, the passivation methods of the present invention are directlyapplicable to the fabrication of a plurality of nanotube fabric baseddevices including, but not limited to, nanotube fabric based sensors,nanotube fabric based field effect transistors, and nanotube fabricbased logic devices.

Passivation Through the Use of Dense Nanotube Fabric Layers

In one aspect of the present disclosure, a nanotube fabric layer ispassivated by increasing the density of the nanotube fabric layer, suchas by adjusting one or more characteristics of the individual nanotubeelements contained in the nanotube fabric layer (e.g., length,orientation, etc.). This increased density limits the porosity of thenanotube fabric layer, thereby limiting the depth to which an adjacentmaterial layer can penetrate. Within this aspect of the presentdisclosure, the density of a nanotube fabric layer can be increased bylimiting the length of the individual nanotube elements comprising thenanotube fabric layer (as depicted in FIG. 4), through the use of aspecific application method—such as, but not limited to, spincoating—which deposits the individual nanotube elements in a densefabric (as depicted in FIG. 5), or through the use of a top layer ofindividual nanotubes specially deposited to provide a densely packedbarrier layer (as depicted in FIG. 6). Each of these methods isdescribed in detail in the following discussion of FIGS. 3-6.

FIG. 3 illustrates a first two terminal nanotube switching device 301comprising a porous nanotube fabric layer 330 deposited over a firstconductive layer 310. Porous nanotube fabric layer 330 is comprised of aplurality of individual nanotube elements 330 a, all of which aresubstantially the same length. A second conductive layer 320 isdeposited over porous nanotube fabric layer 330. As with the twoterminal nanotube switching device depicted in FIG. 2 (200 in FIG. 2),the porosity of the nanotube fabric layer 330 is such that secondconductive layer 320 is permitted to seep through the nanotube fabriclayer 330 and make electrical contact with first conductive layer 310,essentially establishing a short circuit through nanotube fabric layer330.

FIG. 4 illustrates another two terminal nanotube switching device 401wherein the porosity of nanotube fabric layer 430 has been selected bylimiting the length of the plurality of individual nanotube elements 430a within nanotube fabric layer 430. The shorter individual elements 430a form a significantly denser fabric layer 430 (as compared withnanotube fabric layer 330 in FIG. 3) as they are deposited over firstconductive layer 410. In this way, the porosity of nanotube fabric layer430 has been reduced such as to prevent second conductive layer 420 fromseeping through nanotube fabric layer 430 and coming into physical ordirect electrical contact with first conductive layer 410.

Within a non-limiting example, for instance, the individual nanotubeelements 330 a of nanotube fabric layer 330 might be on the order of 1μm, and the individual nanotube elements 430 a of nanotube fabric layer430 might be on the order of 0.4 μm.

FIG. 5 illustrates another two terminal nanotube switching device 501wherein the porosity of nanotube fabric layer 530 has been selectedthrough the use of a specific application method—such as, but notlimited to, spin coating or dip coating—to form nanotube fabric layer530 over first conductive layer 510.

Within a spin coating operation, individual nanotube elements 530 a maybe suspended in a solvent in a soluble or insoluble form and spin-coatedover a surface to generate a nanotube film. In such an arrangement thenanotube fabric layer created may be one or more nanotubes thick,depending on the spin profile and other process parameters. Appropriatesolvents include, but are not limited to: dimethylformamide,n-methylpyrollidinone, n-methyl formamide, ethyl lactate, alcohols,water with appropriate surfactants such as sodium dodecyl sulfate orTRITON X-100, water alone, anisol or other solvents. The nanotubeconcentration and deposition parameters such as surfacefunctionalization, spin-coating speed, temperature, pH and time can beadjusted for controlled deposition of monolayers or multilayers ofnanotubes as required.

The nanotube film could also be deposited by dipping a wafer or asubstrate (such as first conductive layer 510) in a solution of solubleor suspended nanotubes (a dip coating process).

Both spin coating and dip coating allow for the formation of a nanotubefabric layer 530 which is significantly denser—and, in some embodiments,substantially uniform in density—than could be realized with otherdeposition methods. For example, spray coating—wherein a nanotube fabricis formed by spraying a plurality of individual nanotube elements in theform of an aerosol onto a surface—typically yields a nanotube fabriclayer with a non-uniform density and comprising a plurality of voidswhich tend to allow the penetration of adjacent material layers.

In this way, a highly dense nanotube fabric layer 530 deposited througha spin coating or dip coating process will limit the penetration of anadjacent material layer, such as second conductive layer 520.

While the highly dense nanotube fabric layers depicted in FIG. 4 andFIG. 5 (430 and 530, respectively) provide an effective passivationmethod, in some applications such a nanotube fabric layer may proveinconvenient or impractical to produce. A highly dense nanotube fabriclayer (such as 430 in FIG. 4 or 530 in FIG. 5) uses significantly moreindividual nanotube elements than a comparatively less dense fabriclayer of similar geometry (such as 330 in FIG. 3, for example),resulting in significantly higher fabrication costs. Moreover, a denselypacked nanotube fabric layer will tend to possess a very low resistancerange as compared to a less dense fabric layer of similar geometry,significantly limiting, in some applications, the nanotube fabriclayer's usefulness within a two terminal nanotube switch. In someapplications, therefore, a single highly dense nanotube fabric layer maynot provide a complete solution to the problem of adjacent materiallayer penetration. To this end, FIG. 6 illustrates a two terminalnanotube switching device comprising multiple nanotube fabric layers.

Referring now to FIG. 6, a first nanotube fabric layer 630, comprising aplurality of individual nanotube elements 630 a, is deposited over afirst conductive layer 610 in a first operation. In a second operation,a second nanotube fabric layer 640, comprising a plurality of individualnanotube elements 640 a, is deposited over the first nanotube fabriclayer 630. The second nanotube fabric layer is formed with a relativelyhigh density (as described in the discussions of FIGS. 4 and 5 above),preventing second conductive layer 620 from penetrating the secondnanotube fabric layer. In some embodiments of this aspect of the presentdisclosure, second nanotube fabric layer 640 includes a plurality ofrafted nanotube elements. That is, wherein the second nanotube fabriclayer 640 is deposited in such a manner that the individual nanotubeelements 640 a are bundled together along their sidewalls, providing ahighly dense fabric layer.

Within this aspect of the present disclosure, the density of the firstnanotube fabric layer can be selected according to the needs of thespecific application wherein this aspect of the present disclosure isused. In this way a highly dense nanotube fabric layer is used toprevent the second conductive layer 620 from electrically shorting tothe first conductive layer 610 while still preserving the benefits ofusing a comparatively less dense nanotube fabric layer within the twoterminal nanotube switching device.

Passivation Through the Use of a Sacrificial Material

In another aspect of the present disclosure, a nanotube fabric layer ispassivated by using a sacrificial filler material during the fabricationof a two terminal nanotube switching device. Within this aspect of thepresent disclosure, a sacrificial material is deposited over and allowedto penetrate a porous nanotube fabric layer prior to the deposition ofan adjacent material layer. This sacrificial material effectively fillsin the pores of the nanotube fabric layer, preventing the adjacentmaterial layer from penetrating the nanotube fabric layer during thefabrication process. Once the adjacent material layer is formed, thesacrificial material is etched away, allowing the nanotube fabric layerto function within the two terminal nanotube switching device.

FIGS. 7A-7J illustrate a method of passivating a nanotube fabric layerthrough the use of a sacrificial filler material.

Referring now to FIG. 7A, in a first process step 700 a first conductivelayer 710 is provided. Referring now to FIG. 7B, in a second processstep 701 a porous nanotube fabric layer 730 is deposited over the firstconductive layer 710. Referring now to FIG. 7C, in a third process step702 a sacrificial material 740—such as, but not limited to, phosphosilicate glass (PSG) oxide, spin on glass, a physical vapor deposition(PVD) of germanium, or a sacrificial polymer—is flowed over the porousnanotube fabric layer 730 such that it substantially permeates porousnanotube fabric layer 730 forming combined nanotube fabric/sacrificialmaterial layer 730′. Referring now to FIG. 7D, in a fourth process step703, excess sacrificial material 740 is etched (e.g., dry etching suchas reactive ion etching) to remove any material overflowing the top oflayer 730′.

It should be noted that while FIG. 7C (and subsequent figures) depictssacrificial material 740 as essentially completely penetrating nanotubefabric layer 730, the methods of the present disclosure are not limitedin this regard. Indeed, as will become evident in the followingdescription of this aspect of the present disclosure, the depth to whichthe sacrificial material 740 penetrates the nanotube fabric layer 730 isnot important so long as the sacrificial material 740 forms a barrierwithin the nanotube fabric layer which prevents an adjacent materiallayer from penetrating. Similarly, while FIG. 7C depicts sacrificialmaterial 740 overflowing the nanotube fabric layer (necessitatingprocess step 703 depicted in FIG. 7D), the methods of the presentdisclosure are not limited in this regard. Indeed, it will be obvious tothose skilled in the art that in some applications a sacrificialmaterial could be deposited in such a way that it does not overflownanotube fabric layer 730, eliminating the need for process step 703.

Referring now to FIG. 7E, in a fifth process step 704, a secondconductive layer 720 is deposited over the combined nanotubefabric/sacrificial material layer 730′. As the sacrificial material 740substantially fills in the pores of the original nanotube fabric layer730, second conductive layer 720 does not seep into the combinednanotube fabric/sacrificial material layer 730′.

Referring now to FIG. 7F, in a sixth process step 705 a hard mask layer750—such as, but not limited to, an amorphous carbon layer—is depositedin such a way as to define a plurality of individual two terminalswitching devices. Referring now to FIG. 7G, in a seventh process step706, an etch process is used to remove those portions of firstconductive layer 710, combined nanotube fabric/sacrificial materiallayer 730′, and second conductive layer 720 not covered by hard masklayer 750. In this way, three individual nanotube switching devices 760a, 760 b, and 760 c are realized, each comprising a first conductivelayer 710′, a patterned combined nanotube fabric/sacrificial materiallayer 730″, and a second conductive layer 720′. It should be noted thatin some embodiments, sacrificial material 740—in addition to passivatingthe nanotube fabric layer 730 during the fabrication process—may alsoprovide structural integrity to the individual two terminal nanotubeswitching elements 760 a, 760 b, and 760 c during the fabricationprocess. Referring now to FIG. 7H, in an eighth process step 707, thehard mask layer (750 in FIG. 7G) is removed.

Referring now to FIG. 7I, in a ninth process step 708, a wet etchprocess—such as, but not limited to, a hydrofluoric etch—is used tovolatize and remove sacrificial material 740 through the exposed sidesof each combined nanotube fabric/sacrificial material layer 730″ in eachof individual nanotube switching elements 760 a, 760 b, and 760 c. Inthis way each of the individual nanotube switching devices (760 a, 760b, and 760 c) is left with a patterned passivated nanotube fabric layer730′″. Referring now to FIG. 7J, in a final process step 709, adielectric material 770—such as, but not limited to, silicon nitride(SiN)—is deposited over the individual nanotube switch elements 760 a,760 b, and 760 c.

In this way, a sacrificial material is flowed over a nanotube fabriclayer and is used to passivate—as well as, in some embodiments, providestructural support for—the nanotube fabric layer as it is etched intoindividual narrow blocks to form a plurality of two terminal nanotubeswitch elements.

FIGS. 8A-8K illustrate another method of passivating a nanotube fabriclayer through the use of a sacrificial filler material wherein thesacrificial material is volatized and removed through a non-hermeticmaterial layer.

Referring now to FIG. 8A, in a first process step 800 a first conductivelayer 810 is provided. Referring now to FIG. 8B, in a second processstep 801 a porous nanotube fabric layer 830 is deposited over the firstconductive layer 810. Referring now to FIG. 8C, in a third process step802 a sacrificial material 840—such as, but not limited to, phosphosilicate glass (PSG) oxide, spin on glass, a physical vapor deposition(PVD) of germanium, or a sacrificial polymer—is flowed over the porousnanotube fabric layer 830 such that it substantially permeates porousnanotube fabric layer 830. Referring now to FIG. 8D, in a fourth processstep 803, the combined nanotube fabric/sacrificial material layer 830′is etched to remove any material overflowing the top of the layer 830′.

It should be noted that while FIG. 8C (and subsequent figures) depictssacrificial material 840 as essentially completely penetrating nanotubefabric layer 830, the methods of the present disclosure are not limitedin this regard. Indeed, as will become evident in the followingdescription of this aspect of the present disclosure, the depth to whichthe sacrificial material 840 penetrates the nanotube fabric layer 830 isnot important so long as the sacrificial material 840 forms a barrierwithin the nanotube fabric layer which prevents an adjacent materiallayer from penetrating. Similarly, while FIG. 8C depicts sacrificialmaterial 840 overflowing the nanotube fabric layer (necessitatingprocess step 803 depicted in FIG. 8D), the methods of the presentdisclosure are not limited in this regard. Indeed, it will be obvious tothose skilled in the art that in some applications a sacrificialmaterial could be deposited in such a way that it does not overflownanotube fabric layer 830, eliminating the need for process step 803.

Referring now to FIG. 8E, in a fifth process step 804, a non-hermeticconductive layer 820—that is a conductive layer which is substantiallyporous, such as, but not limited to, a physical vapor deposition (PVD)of titanium nitride (TiN), tantalum nitride (TaN), a tungsten (W), or atungsten nitride (WN)—is deposited over the combined nanotubefabric/sacrificial material layer 830′. As the sacrificial material 840substantially fills in the pores of the original nanotube fabric layer830, the non-hermetic conductive layer 820 does not seep into thecombined nanotube fabric/sacrificial material layer 830′.

Referring now to FIG. 8F, in a sixth process step 805 a hard mask layer850—such as, but not limited to, an amorphous carbon layer—is depositedin such a way as to define a plurality of individual two terminalswitching devices. Referring now to FIG. 8G, in a seventh process step806, an etch process is used to remove those portions of firstconductive layer 810, combined nanotube fabric/sacrificial materiallayer 830′, and non-hermetic conductive layer 820 not covered by hardmask layer 850. In this way, three individual nanotube switching devices860 a, 860 b, and 860 c are realized, each comprising a first conductivelayer 810′, a patterned combined nanotube fabric/sacrificial materiallayer 830″, and a non-hermetic conductive layer 820′. It should be notedthat in some embodiments, sacrificial material 840—in addition topassivating the nanotube fabric layer 830 during the fabricationprocess—may also provide structural integrity to the individual twoterminal nanotube switching elements 860 a, 860 b, and 860 c during thefabrication process. Referring now to FIG. 8H, in an eighth process step807, the hard mask layer (850 in FIG. 8G) is removed.

Referring now to FIG. 8I, in an ninth process step 808, a dielectricmaterial 870—such as, but not limited to, silicon nitride (SiN)—isdeposited over the individual nanotube switch elements 860 a, 860 b, and860 c. As the sacrificial material 840 substantially fills in the poresof the original nanotube fabric layer 830, the dielectric material 870does not encroach into the sides of the combined nanotubefabric/sacrificial material layer 830″ within each of the nanotubeswitching devices 860 a, 860 b, and 860 c.

Referring now to FIG. 8J, in a tenth process step 809, a via (880 a, 880b, and 880 c) is formed through dielectric material 870 over each of theindividual nanotube switching elements (860 a, 860 b, and 860 c,respectively) such as to expose the non-hermetic conductive layer 820′of each element. Referring now to FIG. 8K, in a final process step 811,a wet etch process—such as, but not limited to, a hydrofluoric etch—isused to volatize the sacrificial material 840 through the non-hermeticconductive layer 820′ within each of the nanotube fabric layers 830″ ineach of individual nanotube switching elements (860 a, 860 b, and 860c). The porous nature of the non-hermetic conductive layers 820′ allowsthe wet etch process to access the combined nanotube fabric/sacrificialmaterial layer 830″ within each of the individual nanotube switchingdevices (860 a, 860 b, and 860 c) and dissolve and remove sacrificialmaterial 840. In this way each of the individual nanotube switchingdevices (860 a, 860 b, and 860 c) is left with a patterned passivatednanotube fabric layer 830′″.

In this way, a sacrificial material is flowed over a nanotube fabriclayer and is used to passivate—as well as, in some embodiments, providestructural support for—the nanotube fabric layer as it is etched intoindividual narrow blocks to form a plurality of two terminal nanotubeswitch elements.

FIGS. 9A-9M illustrate a method of passivating a nanotube fabric layerthrough the use of a sacrificial filler material wherein the sacrificialmaterial is volatized during the formation of individual two terminalnanotube switching elements.

Referring now to FIG. 9A, in a first process step 901 a first conductivelayer 910 is provided. Referring now to FIG. 9B, in a second processstep 902 a porous nanotube fabric layer 930 is deposited over the firstconductive layer 910. Referring now to FIG. 9C, in a third process step903 a sacrificial material—such as, but not limited to, phospho silicateglass (PSG) oxide, spin on glass, a physical vapor deposition (PVD) ofgermanium, or a sacrificial polymer—is flowed over the porous nanotubefabric layer 930 such that it substantially permeates porous nanotubefabric layer 930, forming combined nanotube fabric/sacrificial materiallayer 930′.

It should be noted that while FIG. 9C (and subsequent figures) depictsthe sacrificial material as essentially completely penetrating combinednanotube fabric/sacrificial material layer 930′, the methods of thepresent disclosure are not limited in this regard. Indeed, as willbecome evident in the following description of this aspect of thepresent disclosure, the depth to which the sacrificial materialpenetrates the combined nanotube fabric/sacrificial material layer 930is not important so long as the sacrificial material 940 forms a barrierwithin the nanotube fabric layer which prevents an adjacent materiallayer from penetrating.

Referring now to FIG. 9D, in a fourth process step 904, a secondconductive layer 920 is deposited over the combined nanotubefabric/sacrificial material layer 930′. As the sacrificial materialsubstantially fills in the pores of the original nanotube fabric layer930, second conductive layer 920 does not seep into the combinednanotube fabric/sacrificial material layer 930′.

Referring now to FIG. 9E, in a fifth process step 905 a first hard masklayer 950—such as, but not limited to, an amorphous carbon layer—isdeposited in such a way as to define a plurality of narrow strips.Referring now to FIG. 9F, in a sixth process step 906, an etch processis used to remove those portions of first conductive layer 910, combinednanotube fabric/sacrificial material layer 930′, and second conductivelayer 920 not covered by first hard mask layer 950. In this way, threelong and narrow strips 960 a, 960 b, and 960 c are realized, eachcomprising a first conductive layer 910′, a patterned combined nanotubefabric/sacrificial material layer 930″, and a second conductive layer920′. It should be noted that in some embodiments, the sacrificialmaterial—in addition to passivating the nanotube fabric layer 930 duringthe fabrication process—may also provide structural integrity to theintermediate structures formed during the fabrication process (that is,long narrow strips 960 a, 960 b, and 960 c).

Referring now to FIG. 9G, in a seventh process step 907, the first hardmask layer (950 in FIG. 9F) is removed. Referring now to FIG. 9H, in aneighth process step 908, a dielectric material 970—such as, but notlimited to, silicon nitride (SiN)—is deposited over long and narrowstrips 960 a, 960 b, and 960 c.

Referring now to FIG. 9I, in a ninth process step 909 a second hard masklayer 950′—such as, but not limited to, an amorphous carbon layer—isdeposited in such a way as to define a plurality of individual nanotubeswitching elements. Referring now to FIG. 9J, in a tenth process step911, an etch process is used to remove those portions of firstconductive layer 910′, combined nanotube fabric/sacrificial materiallayer 930″, and second conductive layer 920′ not covered by second hardmask layer 950′. In this way, a plurality of individual two terminalnanotube switching elements 960 a′, 960 b′, and 960 c′-960 c′″″(additional nanotube switching elements in line with elements 960 a′ and960 b′ and analogous to elements 960 c″-960 c′″″ are not visible in FIG.9J, but would be present) are formed from three long and narrow strips960 a, 960 b, and 960 c, each comprising a first conductive layer 910″,a patterned combined nanotube fabric/sacrificial material layer 930′″,and a second conductive layer 920′. Each row of two terminal nanotubeswitching elements 960 a′, 960 b′, and 960 c′-960 c′″″ remains coated ina layer of etched dielectric material 970′.

Referring now to FIG. 9K in an eleventh process step 912, the secondhard mask layer (950′ in FIG. 9J) is removed.

Referring now to FIG. 9L, in a twelfth process step 913, a wet etchprocess—such as, but not limited to, a hydrofluoric etch—is used tovolatize and remove the sacrificial material through the exposed sidesof each combined nanotube fabric/sacrificial material layer 930′″ ineach of individual nanotube switching elements 960 a′, 960 b′, and 960c′-960 c′″″. In this way each of the individual nanotube switchingdevices (960 a′, 960 b′, and 960 c′-960 c′″″) is left with a patternedpassivated nanotube fabric layer 930″″.

Referring now to FIG. 9M, in a final process step 914, a dielectricmaterial 970″—such as, but not limited to, silicon nitride (SiN)—isdeposited over individual nanotube switching elements 960 a′, 960 b′,and 960 c′-960 c′″″.

In this way, a sacrificial material is flowed over a nanotube fabriclayer and is used to passivate—as well as, in some embodiments, providestructural support for—the nanotube fabric layer as it is etched intoindividual narrow blocks to form a plurality of two terminal nanotubeswitch elements.

Passivation Through the Use of a Non-Sacrificial Material

In another aspect of the present disclosure, a nanotube fabric layer ispassivated by using a non-sacrificial filler material. Within thisaspect of the present disclosure, a non-sacrificial material isdeposited over and allowed to penetrate a porous nanotube fabric layerprior to the deposition of an adjacent material layer. Thisnon-sacrificial material effectively forms a barrier within the nanotubefabric layer, preventing an adjacent material layer from penetratingcompletely through the nanotube fabric layer during the fabricationprocess. The non-sacrificial filler material is selected or deposited insuch a way as it does not adversely affect the switching function of thenanotube fabric layer. As such, a separate fabrication process step toremove the non-sacrificial material is not required.

FIGS. 10A-10D illustrate a method of passivating a nanotube fabric layerthrough the use of a non-sacrificial filler material.

Referring now to FIG. 10A, in a first process step 1001 a firstconductive layer 1010 is provided. Referring now to FIG. 10B, in asecond process step 1002 a porous nanotube fabric layer 1030 isdeposited over the first conductive layer 1010. Referring now to FIG.10C, in a third process step 1003 a filler material 1040—such as, butnot limited to, amorphous carbon, silicon dioxide (SiO₂), and siliconnitride (SiN)—is flowed over the porous nanotube fabric layer 1030 suchthat it flows into and forms a barrier within nanotube fabric layer1030.

Referring now to FIG. 10D, in a fourth process step 1004, a secondconductive layer 1020 is deposited over nanotube fabric layer 1030.While nanotube fabric layer 1030 permits second conductive layer 1020 toseep through the pores and voids present within the porous nanotubefabric layer 1030, the layer of filler material 1040 within the nanotubefabric layer 1030 prevents the second conductive layer 1020 from cominginto physical (and electrical) contact with first conductive layer 1010.

It should be noted that while FIGS. 10C and 10D depict filler material1040 as forming a barrier layer along the bottom of nanotube fabriclayer 1030 and adjacent to first conductive layer 1010, the methods ofthe present disclosure are not limited in this regard. Indeed, as willbecome evident in the following description of this aspect of thepresent disclosure, the depth to which the sacrificial material 1040penetrates the nanotube fabric layer 1030 is not important so long asthe filler material 1040 forms a barrier within the nanotube fabriclayer which prevents an adjacent material layer from penetratingcompletely and coming into physical contact with first conductiveelectrode 1010.

FIGS. 11A-11F illustrate a method of passivating a nanotube fabric layerthrough the use of a porous dielectric material. The formation andapplication of a porous dielectric material (such as, but not limitedto, silicon dioxide aerogel and porous silica) are well known to thoseskilled in the art. Typically a dielectric material is infused withparticles of a second material (typically an organic material) commonlytermed a “porogen” by those skilled in the art. An etching process or ananneal process is typically employed to remove the porogen materialafter the dielectric material has been deposited, leaving behind aplurality of voids or pores within the formed dielectric material layer.

Referring now to FIG. 11A, in a first process step 1101, a firstconductive layer 1110 is provided. Referring now to FIG. 11B, in asecond process step 1102, a nanotube fabric layer 1130 comprising aplurality of individual nanotube elements 1130 a is formed over firstconductive layer 1110. Referring now to FIG. 11C, in a third processstep 1103, a dielectric material 1140 infused with a plurality ofporogens 1140 a is applied and allowed to penetrate nanotube fabriclayer 1130. Referring now to FIG. 11D, in a fourth process step 1104 thedielectric material layer 1140 is etched to remove any materialoverflowing the top of nanotube fabric layer 1130.

Referring now to FIG. 11E, in a fifth process step 1105 an etchingprocess is used to volatize and remove the plurality of porogens 1140 awithin porous dielectric material 1140. The volatizing and removal ofporogens 1140 a within process step 1105 results in a plurality of voidsor pores 1140 b within porous dielectric material 1140.

In a sixth process step 1106, a second conductive layer 1120 isdeposited over the nanotube fabric layer 1130. The porous nanotubefabric layer 1130, now at least partially infused within the porousdielectric material 1140, is substantially passivated, and secondconductive layer 1120 does not encroach the nanotube fabric layer 1130.

The plurality of voids 1140 b within porous dielectric material 1140allows for a plurality of individual nanotube junctions 1130 b (that is,the areas within the nanotube fabric layer where two or more individualnanotube elements meet) to operate freely. Thus, while the nanotubefabric layer 1130 is substantially passivated and does not permitconductive layer 1120 to penetrate through, the overall switchingoperation is still able to function within the void areas 1140 b.

FIGS. 12A-12F illustrate a method of passivating a nanotube fabric layerthrough the use of a room temperature chemical vapor deposition (RTCVD)process.

Referring now to FIG. 12A, in a first process step 1201 a firstconductive layer 1210 is provided. Referring now to FIG. 12B, in asecond process step 1202 a porous nanotube fabric layer 1230 comprisinga plurality of individual nanotube elements 1230 a is deposited overfirst conductive layer 1210. Referring now to FIG. 12C, in a thirdprocess step 1203 a gaseous filler material 1240—such as, but notlimited to, tetraethyl orthosilicate (TEOS)—is flowed over nanotubefabric layer 1240 at room temperature. Referring now to FIG. 12D, in afourth process step 1204 ultraviolet (UV) radiation 1250 is used toconvert the gaseous filler material (1240 in FIG. 12C) into a liquidfiller material 1240′.

Referring now to FIG. 12E, in a fifth process step 1205 the liquidfiller material (1240′ in FIG. 12D) is allowed to penetrate nanotubefabric layer 1230, and an anneal process is used to convert the liquidfiller material (now in place within the nanotube fabric layer 1230)into a solid state 1240″. In this way, a filler material 1240″ forms abarrier layer within nanotube fabric layer 1230, essentially passivatingnanotube fabric layer 1230 as will be shown in final process step 1206(depicted in FIG. 12F) below.

Referring now to FIG. 12F, in a final process step 1206 a secondconductive layer 1220 is deposited over nanotube fabric layer 1230.While second conductive layer 1220 penetrates partially through nanotubefabric layer 1230, filler material 1240″ prevents it from coming intophysical or electrical contact with first conductive layer 1210.

FIGS. 13A-13E illustrate a method of passivating a nanotube fabric layerthrough the use of a non-directional sputter deposition process.

Referring now to FIG. 13A, in a first process step 1301 a firstconductive layer 1310 is provided. Referring now to FIG. 13B, in asecond process step 1302 a porous nanotube fabric layer 1330 comprisinga plurality of individual nanotube elements 1330 a is deposited overfirst conductive layer 1310. Referring now to FIG. 13C, in a thirdprocess step 1303 filler material particles 1340 b are deposited overthe top of nanotube fabric layer 1330 via a non-directional sputterdeposition process.

Sputter deposition processes are well known to those skilled in the art.As depicted in exemplary process step 1303 in FIG. 13C, a wafer offiller material 1340—such as, but not limited to, titanium nitride(TiN)—is bombarded with ions 1350—such as, but not limited to argon(Ar). This bombardment ejects particles 1340 b (or in some casesindividual atoms) of the filler material 1340. In a typical directionalsputter process, an electric field is used to direct the trajectory ofthe ejected filler material particles 1340 b. For example, a typicaldirectional sputter process might apply an electric field over thenanotube fabric layer 1330 such that the ejected filler materialparticles 1340 b impacted the nanotube fabric layer substantiallyperpendicular to the layer itself. In this way, a substantial number ofthe filler material particles 1340 b could be expected to penetratethrough the porous nanotube fabric layer 1330.

Within the methods of the present disclosure, however, a non-directionalsputter deposition process is used. That is, the ejected filler materialparticles 1340 b are allowed to fly away from the filler material wafer1340 in random trajectories. In this way, very few of the ejected fillermaterial particles 1340 b will penetrate the porous nanotube fabriclayer 1330 and most of the ejected filler material particles 1340 b willsimply form a layer over the top surface of nanotube fabric layer 1330.

Referring now to FIG. 13D, in a fourth process step 1304 this top layerof ejected filler material particles 1340 b can be seen covering the topof nanotube fabric layer 1330. Referring now to FIG. 13E, in a finalprocess step 1305 a second conductive layer 1320 is deposited overnanotube fabric layer 1330. The layer of ejected filler materialparticles 1340 b serve to limit the encroachment of the secondconductive layer 1320, preventing the layer from seeping into nanotubefabric layer 1330 and coming into physical or electrical contact withfirst conductive layer 1310.

In this way nanotube fabric layer 1330 is passivated with a layer offiller material particles 1340 b deposited via a non-directional sputterdeposition process, preventing the encroachment of an adjacent materiallayer 1320 while preserving the switching function of the nanotubefabric layer 1330 itself.

Passivation Through the Use of Nanoscopic Particles to Limit thePorosity of the Nanotube Fabric Layer

In another aspect of the present disclosure, a passivated nanotubefabric layer is comprised of a first plurality of individual nanotubeelements and a second plurality of nanoscopic particles. The nanoscopicparticles limit the overall porosity of the nanotube fabric layer—whichlimits the degree to which an adjacent material layer can penetrate thenanotube fabric layer—while preserving the switching function of thenanotube fabric layer itself. In this way, the overall density of ananotube fabric layer can be increased without increasing the density ofthe individual nanotube elements.

FIG. 14 illustrates the formation of a two terminal nanotube switchingdevices including a nanotube fabric layer which comprises bothindividual nanotube elements and nanoscopic particles. A first volume1430 of individual nanotube elements 1430 a is combined with a secondvolume 1440 of nanoscopic particles 1440 a to form an applicationsolution 1450 which comprises both individual nanotube elements 1430 aand nanoscopic particles 1440 a. Specific values for the first volume1430 and the second volume 1440 are selected such that a desired ratioof individual nanotube elements 1430 a to nanoscopic particles 1440 a isrealized. For instance, in one non-limiting example an optimal ratio ofindividual nanotube elements 1430 a to nanoscopic particles 1440 a forthe formation of two terminal nanotube switching devices would be 1:1.

In some embodiments it may be desirable to combine both the individualnanotube elements 1430 a and the nanoscopic particles 1440 a into aliquid medium such as, but not limited to, water to form applicationsolution 1450. Once combined, the volume of the liquid medium can thenbe adjusted as to provide a specific concentration (that is the ratio ofindividual nanotube elements and nanoscopic particles to liquid) optimalfor the application of said solution 1450 over first conductive layer1410 to form a combined nanotube fabric/nanoscopic particle layer 1460(as depicted by structure 1401). Further, in some embodimentsapplication solution 1450 is applied to first conductive layer 1410 viaa spin coating process. However, the methods of the present disclosureare not limited in this regard. Indeed, application solution 1450 couldbe applied through a plurality of methods including, but not limited to,spray coating.

As depicted in structure 1401, application solution 1450 is depositedover first conductive layer 1410 to form composite switching layer 1460.Within the methods of the present disclosure, nanoscopic particles 1440a serve to limit the porosity between individual nanotube elements 1430a within composite switching layer 1460. As such, as second conductivelayer 1420 is applied over composite switching layer 1460—resulting instructure 1402—the encroachment of second conductive layer 1420 intocomposite switching layer 1460 is significantly limited, preventingsecond conductive layer 1420 from seeping through composite switchinglayer 1460 and making physical or electrical contact with firstconductive layer 1410.

In one embodiment of this aspect of the present disclosure, nanoscopicparticles 1440 a are a colloidal dispersion of inert (that is,non-conductive) silica particles. However, the methods of the presentdisclosure are not limited in this regard. Indeed, nanoscopic particles1440 a can take a plurality of forms depending on the needs of anapplication or structure in which the methods of the present disclosureare employed. The nanoscopic particles 1440 a may be spherical, oblong,square, irregular, or any other shapes as would be readily apparent tothose skilled in the art. The nanoscopic particles 1440 a may have atleast one dimension that is in the nanometer size. For example, thenanoscopic particles 1440 a may have at least one dimension which isless than 1000 nm, 500 nm, 400 nm, 300 nm, 200 nm, 100 nm, 50 nm, 40 nm,30 nm, 20 nm, 10 nm, 5 nm, or 1 nm. In certain embodiments, thenanoscopic particles 1440 a may be individual atoms, molecules, or ions.

Nanoscopic particles 1440 a can interact covalently or non-covalently toanother nanoscopic material, for example, carbon nanotubes. In certainembodiments, the nanoscopic particles 1440 a may be miscible with thenanotube elements 1430 a and form a continuous material around theindividual nanotube elements 1430 a. In some other embodiments, thenanoscopic particles 1440 a may be inert to the nanotube elements 1430 aand remain in the same form as initially introduced into the mixture1450 and therefore non-miscible. In yet some other embodiments, thenanoscopic particles 1440 a may be partially miscible with the nanotubeelements 1430 a and form a semi-miscible mixture with the nanotubes.

Furthermore, in certain embodiments, the choice of such nanoscopicparticles 1440 a can include a material or materials that can be formedwith a uniform particle size. In certain applications, the choice of ananoscopic particle can include a material or materials which can befabricated as individual particles within certain dimensions. Forexample, an application may require a nanoscopic particle whereinindividual particles are not larger than some fraction of a devicefeature size.

The choice of such nanoscopic particles can include any material ormaterials which do not adversely affect the switching operation (thatis, the changing from one nominal nonvolatile resistive state toanother) of the composite article. In fact, in certain embodiments, thenanoscopic particles 1440 a may improve switching operation by loweringthe voltage needed for the composite article to change its resistance.

In some other embodiments, inorganic nanoparticles can be utilized. Forexample, silicon based materials (such as, but not limited to siliconoxide and silicon nitride) can be used for said nanoscopic particles1440 a.

In some embodiments, one or more non-conductive allotropes of carbon(such as, but not limited to, diamond, carbon black, and fullerenes) canbe used for said nanoscopic particles 1440 a.

In certain embodiments, nanoscopic particles 1440 a can include amixture of different nanoscopic materials, such as any combination ofnanoscopic particles 1440 a described above.

The nanoscopic particles 1440 a can be obtained by numerous differentways. For example, carbon particles having substantially uniform volumecan be obtained through the process described below. Methods forobtaining other desired nanoscopic materials 1440 a will be readilyapparent to those skilled in the art.

-   -   In a first processing step, reacting a volume of carbon black        material with an oxidizing agent (such as, but not limited to,        nitric acid) to form a carbon slurry in order to decrease the        size of carbon black particles and further remove any metallic        contaminants (via solubilization). The first processing step may        be aided by further introducing other acids, such as        hydrochloric acid.    -   In a next processing step, filtering the carbon slurry formed in        the first process step at low pH (for example, but not limited        to, via cross-flow membranes) to remove any solubilized        impurities    -   In a next processing step, increasing pH level of the carbon        slurry to realize a homogeneous and stable colloidal system (in        some operations, a sonication process may be used to improve        homogeneity)    -   In a next processing step, filtering the realized homogeneous        and stable colloidal system through a train of filters to remove        any particles which could lead to defects in the spin coated        film (in some operations, for example, said system would be        passed through filters with pores as small as 10 nm or 5 nm or        other filters with the smallest pore size available)

FIGS. 15A-15F illustrate a method of further passivating a nanotubefabric layer comprised of individual nanotube elements and nanoscopicparticles through the use of a room temperature chemical vapordeposition (RTCVD) process.

Referring now to FIG. 15A, in a first process step 1501 a firstconductive layer 1510 is provided. Referring now to FIG. 15B, in asecond process step 1502 a porous composite switching layer 1530comprising a first plurality of individual nanotube elements 1530 a anda second plurality of nanoscopic particles 1530 b is deposited overfirst conductive layer 1510. Referring now to FIG. 15C, in a thirdprocess step 1503 a gaseous filler material 1540—such as, but notlimited to, tetraethyl orthosilicate (TEOS)—is flowed over compositeswitching layer 1530 at room temperature. Referring now to FIG. 15D, ina fourth process step 1504 ultraviolet (UV) radiation 1550 is used toconvert the gaseous filler material (1540 in FIG. 15C) into a liquidfiller material 1540′.

Referring now to FIG. 15E, in a fifth process step 1505 the liquidfiller material (1540′ in FIG. 15D) is allowed to penetrate compositeswitching layer 1530, and an anneal process is used to convert theliquid filler material (now in place within the composite switchinglayer 1530) into a solid state 1540″. In this way, a filler material1540″ forms a barrier layer within composite switching layer 1530,essentially passivating composite switching layer 1530 as will be shownin final process step 1506 (depicted in FIG. 15F) below.

Referring now to FIG. 15F, in a final process step 1506 a secondconductive layer 1520 is deposited over composite switching layer 1530.While second conductive layer 1520 penetrates partially throughcomposite switching layer 1530, this encroachment is limited by thepresence of nanoscopic particles 1530 b (as described in the discussionof FIG. 14 above), and the barrier formed by filler material 1540″further prevents second conductive layer 1520 from coming into physicalor electrical contact with first conductive layer 1510.

FIGS. 16A-16E illustrate a method of further passivating a nanotubefabric layer comprising individual nanotube elements and nanoscopicparticles through the use of a non-directional sputter depositionprocess.

Referring now to FIG. 16A, in a first process step 1601 a firstconductive layer 1610 is provided. Referring now to FIG. 16B, in asecond process step 1602 a porous composite switching layer 1630comprising a first plurality of individual nanotube elements 1630 a anda second plurality of nanoscopic particles 1630 b is deposited overfirst conductive layer 1610. Referring now to FIG. 16C, in a thirdprocess step 1603 filler material particles 1640 b are deposited overthe top of composite switching layer 1630 via a non-directional sputterdeposition process.

As discussed in the detailed description of FIGS. 13A-13E, sputterdeposition processes are well known to those skilled in the art. A waferof filler material 1640—such as, but not limited to, titanium nitride(TiN)—is bombarded with ions 1650—such as, but not limited to argon(Ar). This bombardment ejects particles 1640 a (or in some casesindividual atoms) of the filler material 1640. In a typical directionalsputter process, an electric field is used to direct the trajectory ofthe ejected filler material particles 1640 b. For example, a typicaldirectional sputter process might apply an electric field over thecomposite switching layer 1630 such that the ejected filler materialparticles 1640 b impacted the nanotube fabric layer substantiallyperpendicular to the layer itself. In this way, a substantial number ofthe filler material particles 1640 b could be expected to penetratethrough the porous composite switching layer 1630.

Within the methods of the present disclosure, however, a non-directionalsputter deposition process is used. That is, the ejected filler materialparticles 1640 b are allowed to fly away from the filler material wafer1640 in random trajectories. In this way, very few of the ejected fillermaterial particles 1640 b will penetrate the porous composite switchinglayer 1630 and most of the ejected filler material particles 1640 b willsimply form a layer over the top surface of composite switching layer1630.

Referring now to FIG. 16D, in a fourth process step 1604 this top layerof ejected filler material particles 1640 b can be seen covering the topof composite switching layer 1630. Referring now to FIG. 16E, in a finalprocess step 1605 a second conductive layer 1620 is deposited overcomposite switching layer 1630. While second conductive layer 1620penetrates partially through composite switching layer 1630, thisencroachment is limited by the presence of nanoscopic particles 1630 b(as described in the discussion of FIG. 14 above), the layer of ejectedfiller material particles 1640 b further serves to limit theencroachment of second conductive layer 1620, preventing the layer fromseeping into composite switching layer 1630 and coming into physical orelectrical contact with first conductive layer 1610.

In this way composite switching layer 1630 is passivated with a layer offiller material particles 1640 b deposited via a non-directional sputterdeposition process, preventing the encroachment of an adjacent materiallayer 1620 while preserving the switching function of the compositeswitching layer 1630 itself.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention not be limited by thespecific disclosure herein.

What is claimed is:
 1. A method for forming a multilayer nanotubefabric, comprising: forming a first nanotube application solution, saidfirst nanotube application solution comprising a first plurality ofnanotube elements suspended in a solvent; depositing said first nanotubeapplication solution over a material layer to form a first nanotubefabric layer having a first density; selecting a second plurality ofnanotube elements, said second plurality of nanotube elements having alength below a selected limit; forming a second nanotube applicationsolution, said second nanotube application solution comprising saidselected second plurality of nanotube elements suspended in a solvent;depositing said second nanotube application solution over said firstnanotube fabric layer to form a second nanotube fabric layer with aselected second density, wherein said first nanotube fabric layer andsaid second nanotube fabric layer form a multilayer nanotube fabric; andwherein said second density is selected by selecting the length of saidsecond plurality of nanotube elements, and said second density of saidsecond nanotube fabric layer is sufficient to passivate said multilayernanotube fabric.
 2. The method of claim 1 wherein said selected densitylimits the encroachment of a second material layer formed over oradjacent to said multilayer nanotube fabric into said multilayernanotube fabric.
 3. The method of claim 1 wherein said length of secondplurality of nanotube elements is limited to less than about 500 nm. 4.The method of claim 1 wherein said length of second plurality ofnanotube elements is limited to less than about 100 nm.
 5. The method ofclaim 1 wherein said first nanotube application solution and said secondnanotube application solutions are deposited using one of a spin coatingprocess, a spray coating process, or a dip coating process.
 6. Themethod of claim 1 wherein said nanotube elements are carbon nanotubes.7. The method of claim 1 wherein said solvent is one ofdimethylformamide, n-methylpyrollidinone, n-methyl formamide, ethyllactate, alcohol, or water.
 8. The method of claim 1 wherein said secondnanotube fabric layer has a substantially uniform density.
 9. The methodof claim 1 wherein said first density is selected to increase theswitching function of said first nanotube fabric layer.